I also have the same Apple 20W USB-C Power Adapter and Apple USB-C to lighting cable.
If the battery status start from anywhere between 1-49%, then the charger will use fast mode for 30 mins - that's why you feel heat anywhere warm/hot.
If the battery status is over 50% it will go slow charging I think - that's feel almost no heat.
It's all depend what kind of case you have & room temperature - that also cause over heating issue.
I'm not sure if I'm correct with above facts - just saying it